In this article we will discuss about the definition of gene families.
While most prokaryotic genes are represented only once in the genome, some genes in eukaryotes are repeated many times in multiple copies, called gene families, thus contributing to the large size of the genome. In specific cases, multiple copies of a gene are necessary to produce RNAs or proteins that are required in large quantities, such as ribosomal RNAs and histones.
In other cases, distinct members of a gene family may be transcribed in different tissues or at different stages of development. For instance, the alpha and beta subunits of haemoglobin are both encoded by gene families in the human genome, but different members of these families are expressed in embryonic, foetal and adult tissues. Members of some gene families such as the globin genes are clustered within a region of DNA while other gene families are dispersed on different chromosomes.
Gene families are thought to have arisen by duplication of an original ancestral gene. The different members of the family then diverged during evolution as a consequence of mutation. Such divergence can lead to the evolution of related proteins that can function in different tissues or at different stages of development.
For example, the foetal globins have a higher affinity for O2 than adult globins which allows the foetus to obtain O2 from the maternal circulation. However, not all mutations offer advantage in gene function. The mutations in some gene copies have resulted in their loss of ability to produce a functional gene product.
For example, the human alpha and beta globin genes each contain two genes that have been inactivated by mutations. The non-functional gene copies are called pseudogenes, represent evolutionary relics which increase the size of the eukaryotic genome without making a functional genetic contribution.